Drive systems for machines, including both on-highway and off-highway machines, generally include an internal combustion engine providing power to a transmission, such as an automatic transmission. The transmission is configured to transmit that power, according to a specific gear ratio, to one or more ground engaging elements of the machine. According to a specific example, the drive system may also include a torque converter fluidly coupling the internal combustion engine and the transmission, and one or more axles, differentials, and final drives for transmitting power from the transmission to drive wheels of the machine. In an electronically controlled automatic transmission, an electronic controller may use sensors to detect various operating conditions of the machine, including the internal combustion engine and transmission, in order to control shifting of the transmission gear ratios. Such shifting generally occurs according to predetermined shift points, which may be stored in a memory of the machine. Typically, the shift points are selected to provide a managed change of power as the transmission shifts from one gear ratio to another gear ratio.
Occasionally, a gear hunting, or shift hunting, event may occur, when the transmission selects a new gear ratio and subsequently returns to a previous gear ratio. For example, a gear hunting event may occur due to a loss of machine speed during the shift from the old gear ratio to the new gear ratio, or when there is not enough power to maintain the machine speed in the new gear ratio. To reduce occurrence of such a gear hunting event, an anti-hunt strategy may be employed. For example, U.S. Pat. No. 5,738,605 teaches an anti-hunt strategy for an automatic transmission. Specifically, the control strategy uses learned vehicle inertia, a road load torque, and an expected torque in an upshift gear to determine a projected post shift acceleration. If vehicle speed and throttle position are within an allowable shift zone and if the predicted post shift acceleration exceeds a threshold value, the vehicle automatic transmission is allowed to upshift. Otherwise, the upshift is prevented. It should be appreciated, however, that there remains a continuing need for improved strategies for controlling gear ratio changes in electronically controlled automatic transmissions.
The present disclosure is directed to one or more of the problems set forth above.